Processing chambers and methods to disrupt the boundary layer are described. The processing chamber includes a showerhead and a substrate support therein. The showerhead and the substrate support are spaced to have a process gap between them. In use, a boundary layer is formed adjacent to the substrate support or wafer surface. As the reaction occurs at the wafer surface, reaction products and byproduct are produced, resulting in reduced chemical utilization rate. The processing chamber and methods described disrupt the boundary layer by changing one or more process parameters (e.g., pressure, flow rate, time, process gap or temperature of fluid passing through the showerhead).